Growth Rates of Upper Permian Carbonate Platform, Capitan Margin of Northern Delaware Basin
P. M. Harris, G. A. Grover
Subsurface and outcrop studies of the Capitan margin (late Guadalupian, Capitanian) in the northern Delaware basin have revealed that over 80% of the total progradation of the margin, over half the total aggradation of the platform (150 of 290 m), and over two-thirds of the basin fill (190 of 280 m) occurred during an early phase of development equating with Seven Rivers beds on the platform. The amount of progradation varied from 6 km from outcrop data in the Guadalupe Mountains to 19 km along trend to the east from subsurface information. The later phase of Capitan margin development was coincident with Yates and Tansil deposition on the platform and was dominated by aggradation and steepening of the margin.
Corresponding to this two-phase model, two third-order cycles of relative sea level occur within the Capitanian on the eustasy curve of Ross and Ross: a Seven Rivers cycle lasting 1.5 m.y. and a Yates-Tansill cycle of 1.0 m.y. Progradation rates for the Capitan range from 2.6 to 8.3 m/1,000 years. Similar rates are calculated from high-resolution seismic lines across the Cenozoic margin of northwestern Great Bahama Bank. Accumulation rates for the Capitan, uncorrected for compaction, average 125 µm/year and 335 µm/year for the early phase shelf and shelf margin, respectively, and 160 µm/year and 430 µm/year for the later phase shelf and shelf margin. These accumulation rates are similar to those of other ancient, prograding platforms.
By analogy with the Bahamas example and compatible with Permian sea level curves, the two phases of Capitan growth appear to reflect two cycles with a rising sea level. A slowly rising sea level during the early phase allowed significant off-shelf transport of carbonate debris and siliciclastics to account for the high progradation rates. A faster rate of sea level rise during the later phase resulted in less progradation, entrapment of siliciclastics on the shelf, steepening of the margin, and increased water depth in the basin.
AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.